Flash a new rom on Xperia M with locked bootloader

hello everyone i'm new to this flashing devices thing so be patient with me please :D
i have an Xperia M C1905 with locked bootloader and i can't seem to unlock it but i really want to flash new roms of Android 5 but i have no idea how to do so without CWM so can you please help me by telling me how or showing me a thread that does i will be forever thankful :D ;)